The invention relates to electromagnetic interference (EMI) and, more specifically, to the shielding of electronic systems for the suppression of EMI.
When your older sister""s electric hair dryer interrupted your viewing of xe2x80x9cI Love Lucyxe2x80x9d; that was electromagnetic interference (EMI). Although you may not have thought so at the time, the interference from a cabinet of electronics equipment is a far more serious form of EMI. Various regulatory bodies and technical associations have, over the years, issued increasingly strict regulations and standards regarding the allowable levels of EMI. One such standard, Bellcore GR-1089, which is hereby incorporated by reference, in its level three requirements dictates that EMI shielding doors are an inadequate method of suppressing EMI. That is, for level three compliance, electronic equipment housed within cabinets must not exceed threshold emission levels, even with the cabinet doors, which typically act to suppress EMI, open.
Providing ready access to the racks of electronics cards housed within a cabinet by opening a door, and the suppression of EMI, to some extent, work at cross purposes. There are several approaches to providing EMI shielding, some of which are discussed in U.S. Pat. No. 5,286,318 issued to Sims, et al, and U.S. Pat. No. 6,075,205 issued to Zhang, which are hereby incorporated by reference.
An EMI shielding method and apparatus that provides efficient and substantial shielding for electronics systems would therefore be highly desirable.
An electromagnetic interference (EMI) shielding system in accordance with the principles of the present invention includes one or more electrically conductive gaskets placed between electronic circuit cards in a card rack. One or more such card racks are used to hold circuit cards within an electronics cabinet. The conductive gaskets are affixed to circuit cards to form a substantially continuous conductive shield at the front of a card rack, that is, at the end of the card rack that is nearest the door of the cabinet in which the card racks are mounted. The conductive shield thus formed may be connected to a reference potential, such as cabinet ground, for example.
In an illustrative embodiment, a gasket is attached to each circuit card within a rack, the gasket xe2x80x9cfillingxe2x80x9d the space between the faceplates of two adjacent circuit cards. Each gasket includes a xe2x80x9cknife edgexe2x80x9d, that overlaps a portion of the faceplate of the card to which it is attached. The knife edge eases the insertion and removal of an adjacent circuit card by deflecting the card into it""s proper slot. That is, without the knife edge deflection, the insertion of a neighboring card may tend to tear the gasket from its mount; the knife edge tends to xe2x80x9cshoehornxe2x80x9d the neighboring circuit card into the appropriate, adjacent, card slot. In this illustrative embodiment, the gasket is composed of fabric filled with a resilient, compressible, material and includes a stiffening member that supports the gasket in the lateral direction. The gasket extends the length of the faceplate and is grounded through contact with the faceplate or one or more latches located on the faceplate.
The shape of each gasket is similar to that of a hockey stick, with the narrow xe2x80x9chandlexe2x80x9d of the gasket designed to readily fit in the gap between adjacent circuit card faceplates. The broader section of the gasket is designed to fit snugly compressed between two adjacent circuit card faceplates, exerting force on the faceplates due to the nature of the resilient filling, and to thereby make low-impedance electrical contact along the length of the two faceplates. The xe2x80x9celbowxe2x80x9d of the gasket, previously referred to as a knife edge, assists in the insertion of neighboring circuit cards, as previously described.